Not all postseason moments are the same, but all are different than the regular season -- otherwise, the big leagues wouldn't keep separate statistics for each. And that reason can't be just because of counting stats, because batting averages, OPS and ERA are based on percentages.
In the playoffs, you're facing the best teams, hitters, pitchers, and fielders -- but if that's not always true in the Wild Card Era, then at least each postseason series confronts ballplayers with the do-or-die pressures of elimination, with the season on the line.
If you combined career postseason and regular season stats, certain great players would definitely rise above other really good players who for some reason performed at lesser levels (injuries, sweaty palms, coin flip distractions -- who knows!).